The invention relates to a freeze-resistant metering valve which can be used in automotive engineering, in particular utility vehicles. The freeze-resistant metering valve is, in particular, suitable for exhaust gas after treatment systems and/or exhaust systems.
Motor vehicles, in particular utility vehicles which are intended to be used in regions with a temperate climate or even arctic regions, have to be designed so that they can withstand temperatures below zero degrees Celsius without sustaining damage. This is generally possible by the choice of suitable materials. Alternatively, for many years an additional source of heat has been used when temperatures fall too low.
In order to reduce Nitrogen Oxide (NOx) in the exhaust gas of motor vehicles, in particular diesel vehicles, automobile manufacturers and suppliers have agreed to use a 32.5% urea-water solution (UWS). Due to the high proportion of water in the solution, even at low negative temperatures (degrees Celsius), the solution which is pressurized during operation freezes.
For many years now, the industry has been concerned with how the problem of the freezing of the urea-water solution can be handled. One solution consists of removing all the UWS by means of compressed air when switching off the motor vehicle. Such a system requires the presence of an air compressor on board the vehicle. An air pressure generator is typically incorporated in large utility vehicles. No specific air supply system is provided in small utility vehicles and automobiles which are equipped with a diesel engine.
The costly algorithms with which a control device is to be programmed, so that faulty behavior due to freezing can be identified, can be seen from DE 10256169 A (Toyota Motor Corporation Ltd).
DE 10139139 A (Robert Bosch GmbH) proposes to provide the reducing agent line with electrical heating in order to eliminate freezing of the reducing agent. The fact that this is impractical can be seen from DE 19935920 A (Siemens AG). It can be seen from this publication that the heating power requirement for the reducing agent reservoir alone would exceed one kilowatt. Therefore, it can be further seen from the publication that a heat exchanger can be incorporated. According to DE 10139142 A (Robert Bosch GmbH) the heat exchanger has to prevent freezing, even at temperatures below −11° C. The requirements of automobile manufacturers go even further. They require the valves to work perfectly even at an outside temperature of −40° C. It has been considered, therefore, as in DE 4432577 A (Siemens AG), to incorporate a special back-flow prevention valve with variable control operation. DE 4432576 A (Siemens AG) also refers to the difficulty of using frost protection agents. Operating with different volumes is therefore possible.
What all these solutions have in common is that additional measures have to be taken to overcome the risk of freezing. It would be desirable to have a freeze-resistant metering valve which operates perfectly at the high temperatures of the exhaust gas stream which can exceed 700° C. and is simultaneously freeze-resistant. Even at an outside temperature of −40° C., the metering valve still has to be able to be operated, provided that the UWS is present in liquid form. Therefore, the entire system in which the metering valve is incorporated is to be of energy efficient design.